To achieve next generation high-capacity and long-distance communication systems, a technique for generating modulated optical signals using digital signal processing at a transmitter is being studied. By making use of digital signal processing, not only a modulated optical signal such as a quadrature amplitude modulation (QAM) signal or an orthogonal frequency division multiplexing (OFDM) signal, but also a desired waveform such as a dispersion pre-equalized signal can be produced. Besides, by performing digital signal processing to produce electrical signals for different modulation schemes, modulation can be switched between multiple modulation schemes using a single optical modulator.
In typical optical transmission systems, optical modulators are controlled so as to stabilize signals to be transmitted. A similar stabilization technique is demanded for transmission systems using digital signal processing. One of the known signal stabilization techniques is automatic bias control (ABC), which technique is applied generally to lithium niobate (LiNibO3 abbreviated as LN) modulators. The ABC technique is used to prevent the transmitted signal quality from being degraded due to drift of the bias voltage applied to the LM modulator.
In modulation schemes making use of four or more levels phase shift keying including quadrature phase shift keying (QPSK) or 16-QAM, optical signals modulated by electrical drive signals are produced through an in-phase arm (I-arm) and a quadrature phase arm (Q-arm), respectively. A relative phase difference is provided between the light waves travelling through the I-arm and the Q-arm. In QPSK and 16QAM, a phase shifter is used to provide a phase difference of π/2 radians between the I-arm and the Q-arm. To maintain the π/2 phase difference, automatic bias control is applied to the phase shifter. See, for example, Japanese Laid-open Patent Publication No. 2007-082094.